Contact lenses are cast molded by depositing a curable liquid (a mixture of polymerizable monomers) into a mold cavity formed by two mold halves, curing the liquid, opening the mold cavity, and removing the lens. Other processing steps, for example, hydration in the case of hydrogel lenses, may also be required to produce a finished lens. One mold half forms the anterior lens surface (anterior mold). The other mold half forms the posterior lens surface (posterior mold). The mold halves are typically used only once and then either serve as packaging means or are discarded.
The molds are themselves formed by injection molding. More specifically, tools for the production of molds are formed in accordance with the specification of the corresponding lens surface. The tools are fitted into mold plates in the injection molding machine, and the molds are produced by injection molding a selected resin. Unlike the molds which are used only once, the injection molding tools are used to make thousands of molds. However, the tools must be manufactured to extremely high specifications so that no roughness or surface defects are transferred to the mold being made from the tool. Any such defects on the mold surface would be transferred to, and appear on, the finished lens. The tools are typically made from brass, stainless steel, nickel or some combination thereof.
It is the ultimate desired lens product which determines the design of the mold; more particularly, that portion of the mold which forms a lens surface. The needed mold parameters, in turn, determine the design of the corresponding injection mold tool. According to conventional design procedures, the desired base curve of the lens (a lens parameter related to fitting characteristics) determines the design of the posterior mold. Given the posterior design of the lens (and the corresponding mold and tool), the desired optical characteristics of the lens determines the anterior design of the lens (and the corresponding mold and tool).
Consider, for example, the application of this design approach to hydrogel contact lenses. Hydrogel contact lenses are usually made available in powers of 0.25 diopter increments. Each time a different power lens is to be produced, a corresponding anterior mold type is used. One posterior mold type is used throughout the power range. A contact lens series having powers of, for example, from -0.25 D to -6.00 D in 0.25 D increments has 24 different lens types. Cast molding of the series according to conventional procedures therefore requires 24 differing anterior mold types and one posterior mold type. The number of tools required to produce the series is therefore 25.
Ways have been sought to reduce the number of types of tools required to produce a contact lens series. U.S. Pat. No. 5,200,121 discloses producing only a limited number of tools, but then further modifying the molds produced by the tools to impart the proper power and base curves on finished lenses. A drawback of this method is the need to modify and handle each mold, which is only used once.